Nondestructive Testing and Measurements

Ultrasonic testing is one of the most widely used methods for nondestructive inspection. In plastics, the primary application is the detection of discontinuities and measurement of thickness. Ultrasonic techniques can also be used for determining the moisture content 

Handbook of Plastics Testing and Failure Analysis, Third Edition, by Vishu Shah Copyright 2007 by John Wiley Sons, Inc. 

The term ultrasonic, in a broad sense, is applied to describe sound with a frequency above 20,000 cycles/sec. Commercial ultrasonic testing equipment generally employs the testing frequency in the range from 0.75 to 20 MHz. To provide a basis for understanding the ultrasonic system and how it operates, it is necessary to introduce the following terms  

Frequency Generator. A device that imposes a short burst of high-frequency alternating voltage on a transducer. 

Transducer. A transducer or a probe is a device that emits a beam of ultrasonic waves when bursts of alternating voltage are applied to it. An ultrasonic transducer is comprised of piezoelectric material. Piezoelectric material is material that vibrates mechanically under a varying electric potential and develops electrical potentials under mechanical strain, thus transforming electrical energy into mechanical energy and vice versa (2). As the name implies, an electrical charge is developed by a piezoelectric crystal when pressure is applied to it and reverse is also true. The most commonly encountered piezoelectric materials are quartz, lithium sulfate, and artificial ceramic materials such as barium titanate. 

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Laser-based profilometry systems have also been applied for nondestructive testing and measurement of both smooth-bore and rifled gun tubes. Working through Small Business Innovation Research program, the U.S. Army has developed laser-based profilometry systems for the inspection of the 120mm cannon used on the MI-Al Abrams main battle tank. Systems have also been built to measure the erosion of 25 mm and 155 mm rifled gun tubes. 

NDT techniques are useful in determining the flaws, discontinuities, and joints. In simplest form, nondestructive testing involves measurement, weighing, visual examination. 

Holography makes it possible to use interferometry for measurements on objects with rough surfaces. Holographic interferometry is now a powerful tool for nondestructive testing and strain analysis. 

Doyle, J. L., Wood, G. R., and Bondurant, P. D. Using Laser-Based Profilometry to Locate and Measure Corrosion Fatigue Cracking in Boiler Tubes, Materials Evaluation, D The American Society of Nondestructive Testing, Inc., Vol. 51, No. 5, pp. 556-560 (1993). 

As well as being a good way of measuring the yield strengths of materials like ceramics, as we mentioned above, the hardness test is also a very simple and cheap nondestructive test for (Ty. There is no need to go to the expense of making tensile specimens, and the hardness indenter is so small that it scarcely damages the material. So it can be used for routine batch tests on materials to see if they are up to specification on without damaging them. 

This photoelastic stress analysis is a technique for the nondestructive determination of stress and strain components at any point in a stressed product by viewing a transparent plastic product. If not transparent, a plastic coating is used such as certain epoxy, polycarbonate, or acrylic plastics. This test method measures residual strains using an automated electro-optical system. 

ARCHAOMETRY. Application of chemical and physical analytical methods to archaeology, Among those used are microanalyhcal methods, spectroscopic analysis, X-ray, and other types of nondestructive tests. For age determination 14 C measurement (chemical dating) is one of the most valuable techniques. 

Corrosion detection plays an important role in any corrosion control program. Most of the methods employ nondestructive test methods and include hydrogen evaluation, radiography, dynamic pressure, corrosion probes, strain gauges and eddy current measurements. Of these, the methods employed in cooling tower practice are hydrogen evaluation and corrosion probes. 

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